Recently, small-sized computers such as mini computers and personal computers have been widely employed in business. For this reason, it has become very important to store computer data with high security, and therefore magnetic recording tapes which are named "back-up tapes" have been employed for that purpose. The magnetic recording tapes for computer storage are further required to have great storage capacity so as to store a great amount of computer data with high security and high reliability.
The back-up tape comprises a flexible plastic web (i.e., support) and a magnetic recording layer provided thereon, in the same manner as in the conventional magnetic recording tapes such as an audio tape and a video tape. The magnetic recording layer comprises a ferromagnetic powder and a binder.
The magnetic recording tape is repeatedly run in contact with a magnetic head. The back-up tape sometimes is more repeatedly run than the conventional audio or video tape. The back-up tape further should show high reliability to keep the stored computer data safely under severe environmental conditions such as at a high temperature and a high relative humidity.
The back-up tape is encased in a cassette, in the same manner as for an audio tape. The most easy way to increase the data storage capacity of a magnetic recording tape is to increase the length of the tape. However, the size of the compact cassette for encasing the back-up tape cannot be freely enlarged. Therefore, if the length of the back-tape is desired to increase, the thickness of the tape should be decreased. For this reason, the most back-up tape has a decreased thickness, as compared with the conventional audio tape.
The increased data storage capacity of a magnetic recording tape also can be attained by decreasing the size of the ferromagnetic powder and decreasing the thickness of the magnetic recording layer. The back-up tape also should have improved electromagnetic characteristics, and the improvement of electromagnetic characteristics can be accomplished by giving to the magnetic recording layer high smoothness.
The decreased tape thickness and increased smoothness of the surface of the magnetic recording layer sometimes cause tape running troubles such as wrong tape winding. For the reason, a back-coating layer is generally provided on the reverse side of the back-up tape, that is, on the surface of the support opposite to the surface on which the magnetic recording layer is provided. The back-coating layer naturally should be thin so as not to apparently increase the tape thickness.
U.S. Pat. No. 5,419,961 discloses a magnetic recording tape having a decreased thickness. For instance, it refers to a magnetic recording tape having a total thickness of 10 .mu.m which has a back-coating layer of 0.5 .mu.m, as well as a magnetic recording tape having a total thickness of 9.5 .mu.m which has a back-coating layer of 0.5 .mu.m. For obtaining a magnetic recording tape having improved anti-static property and running performance, the former magnetic recording tape has a back-coating layer containing a carbon black of a relatively small particle size, and the latter magnetic recording tape has a back-coating layer containing a carbon black of a relatively large particle size as well as a carbon black of a relatively small particle size. It describes further that a preferred support web may comprise a polyester, especially polyethylenenaphthalate (PEN).
From the viewpoint of recent development of the back-up tape, namely, a magnetic recording tape for computer data storage, however, the magnetic recording tape disclosed in the above-mentioned United States Patent is not enough in its performance. Mere decrease of the tape thickness cannot produce a back-up tape having satisfactory electromagnetic and running characteristics.